1. Field of the Invention
The present invention relates to a QPSK modulation signal receiving unit suitable for receiving a QPSK modulated signal such as a digital television signal sent from a broadcasting satellite.
2. Description of the Related Art
The construction and operation of a conventional QPSK modulation signal receiving unit will be described with reference to FIG. 5. For example, a QPSK modulation signal such as a digital television signal received by a parabolic antenna (not shown) outdoors is subjected to frequency conversion by an outdoor converter (not shown) mounted on the outdoors and supplied to a band-pass filter 31. The signal is amplified by a variable gain amplifier 32 and the amplified signal is supplied to a mixer 33. The television signal supplied to the mixer 33 is mixed with a local oscillation signal from a local oscillator 34 and frequency converted to an intermediate frequency signal.
The intermediate frequency signal passes through a variable attenuater 35, an intermediate frequency amplifier 36, and an intermediate frequency filter 37, and is supplied to an IQ demodulator 38. The IQ demodulator 38 separates the supplied intermediate frequency signal into an I signal and a Q signal whose phases are different by 90 degrees and the I and Q signals are supplied to an A/D converter 39. The A/D converter 39 converts the I and Q signals into a digital signal and supplies the digital signal to a QPSK demodulator 40. The QPSK demodulator 40 QPSK demodulates the digital signal and supplies the QPSK demodulated signal to an error correction circuit 41.
The gain of the variable gain amplifier 32, the attenuation amount of the variable attenuater 35, and the gain of the IQ demodulator 38 are automatically controlled in accordance with the level of the received QPSK modulation signal. An automatic gain control voltage Vagc for the control is proportional (or inverse proportional) to the level of the QPSK modulation signal and is generated by the QPSK demodulator 40.
The automatic gain control voltage Vagc generated by the QPSK demodulator 40 is supplied to the variable gain amplifier 32, the variable attenuater 35, and the IQ demodulator 38. In this case, it is controlled so that the level of the QPSK modulation signal differs when the gain is changed or the attenuation is started in the variable gain amplifier 32, the variable attenuater 35, and the IQ demodulator 38.
Specifically, when the level of the received QPSK modulation signal becomes high to, for example, minus 70 dBm as a first level, first, attenuation of the gain of the IQ demodulator 38 starts. Consequently, the automatic gain control voltage Vagc is supplied to the IQ demodulator 38 after being amplified by an operational amplifier 42. When the level of the QPSK modulation signal becomes, for example, minus 45 dBm as a second level, the attenuation amount of the variable attenuater 35 starts to be increased. Further, when the level of the QPSK modulation signal rises to, for example, minus 25 dBm as a third level, the gain of the variable gain amplifier 32 starts to be decreased.
FIG. 6 shows the level change characteristics of an output level (indicated by curve X) of the IQ demodulator 38 for an input level (received level) of the QPSK modulation signal, an input level (indicated by curve Y) of the IQ demodulator 38, and an input level (indicated by curve Z) of the mixer 33.
In FIG. 6, since the gain or attenuation control does not operate until the level of the QPSK modulation signal rises to the first level A, each of the output level of the IQ demodulator 38, the input level of the IQ demodulator 38, and the input level of the mixer 33 rises according to the rise in the input level of the QPSK modulation signal. Since the gain or attenuation control of the IQ demodulator 38 starts at a time point when the input level of the QPSK modulation signal reaches the first level A, the output level of the IQ demodulator 38 becomes almost constant (about plus 4 dBm).
At the time point when the input level of the QPSK modulation signal reaches the second level B, the attenuation amount of the variable attenuater 35 starts to increase, so that the input level of the IQ demodulator 38 becomes almost constant (about minus 25 dBm).
At the time point when the input level of the QPSK modulation signal reaches the third level C, reduction in the gain of the variable gain amplifier 32 starts, so that the input level of the mixer 33 becomes almost constant (about minus 15 dBm).
As described above, in the conventional unit, as the level of the QPSK modulation signal rises, the control is performed sequentially from the post stage side to the front stage side in such a manner that the gain of the IQ demodulator 38 starts to attenuate first, the attenuation amount of the variable attenuater 35 starts to increase and, finally, the gain of the variable gain amplifier 32 starts to be reduced.
In the conventional receiving unit, however, until the input level of the QPSK modulation signal rises to the third level C, the input level of the mixer 33 rises in association with the rise in the level of the QPSK modulation signal (refer to the curve Z).
On the other hand, among a number of QPSK modulation signals, some QPSK modulation signals have a low bit rate and some QPSK modulation signals have a high bit rate. The signal density of the QPSK modulation signal having a low bit rate is higher than that of the QPSK modulation signal having a high bit rate.
The input level of the QPSK modulation signal supplied to the mixer 33 is therefore high until the input level of the QPSK modulation signal reaches the third level. The QPSK modulation signal supplied to the mixer 33 is leaked to the local oscillator 34 via the mixer 33. The operation of the local oscillator 34 consequently becomes unstable due to the leaked QPSK modulation signal and phase noise of the local oscillation signal increases. As a result, a bit error increases and, moreover, a problem such that the picture quality deteriorates occurs.
It is therefore an object of the invention to provide a receiving unit in which phase noise of a local oscillation signal is not increased even when a QPSK modulation signal having a low bit rate is supplied.
According to the invention, in order to solve the problems, there is provided a QPSK modulation signal receiving unit comprising: a first level changing means for receiving a QPSK modulation signal, changing the level of the received QPSK modulation signal, and outputting a resultant signal; a mixer for frequency converting the QPSK modulation signal outputted from the first level changing means and outputting an intermediate frequency signal; a second level changing means for changing the level of the intermediate frequency signal and outputting a resultant signal; an IQ demodulator for demodulating the intermediate frequency signal outputted from the second level changing means, outputting a first intermediate frequency signal and a second intermediate frequency signal whose phases are different from each other by 90 degrees, changing the levels of the first and second intermediate frequency signals, and outputting resultant signals; and a QPSK demodulator for demodulating the first and second intermediate frequency signals outputted from the IQ demodulator and outputting a QPSK signal, wherein reduction in output level of the IQ demodulator starts when the QPSK modulation signal is at the first level, reduction in output level of the first level changing means starts when the QPSK modulation signal is at the second level higher than the first level, and reduction in output level of the second level changing means starts when the QPSK modulation signal is at the third level higher than the second level.
According to the QPSK modulation signal receiving unit of the invention, a control voltage for changing the output levels of the first level changing means, the second level changing means, and the IQ demodulator is generated by the QPSK demodulator, the first level changing means is constructed by a first variable attenuater comprising first, second, and third pin diodes which are connected in series, the second level changing means is constructed by a second variable attenuater comprising fourth and fifth pin diodes which are connected in series, a current flowing through the first to fifth pin diodes is controlled by the control voltage, and the current is decreased in association with the rise in the level of the QPSK modulation signal.
According to the QPSK modulation signal receiving unit of the invention, an amplifier for amplifying the QPSK modulation signal is provided, and the first variable attenuater is provided at the next stage of the amplifier.